The present invention is directed to the field of electronic devices, such as piezoelectric devices. The term “piezoelectric device” includes resonators, filters and surface acoustic wave devices. More particularly, the present invention is directed to a hermetic silicon package for such devices and a method of making a surface mountable electronic component utilizing such a package.
For over 50 years, the assignee of this patent application has provided piezoelectric devices such as quartz resonators, packaged in a glass housing, for use in aerospace applications. For this particular application, it is important that the piezoelectric device be housed in a vacuum chamber to prevent contamination of the quartz crystal since contaminants can cause deterioration of the accuracy of the resonator. While this deterioration is slow and typically involves changes in accuracy in the parts per billion per day range, for aerospace applications where precision accuracy is essential and the resonator needs to operate for 10–50 years on some exploratory missions, any deterioration is detrimental to the function of the resonator and, hence, to the space craft.
With the advent of solid state electronic circuitry, the number of companies manufacturing glass tubes or “bubbles” (of the type previously used for electronic vacuum tubes) has continued to dwindle, commensurate with the demand for their products. For some applications, the glass bubble can be replaced by a copper housing. However, for aerospace environment where the piezoelectric device may be exposed to temperatures exceeding 300° C., copper packages are subject to collapse as the metal reaches a softening point and the internal vacuum may draw contaminants into the cavity. This permits the piezoelectric device housed therein to be subject to whatever contaminants may be contained in the ambient environment, leading to degradation of its properties.
Within the past fifteen years, Sandia National Laboratory has performed some work on a silicon package which may be useful in housing piezoelectric elements for certain applications. This silicon package is taught in U.S. Pat. Nos. 5,198,716 and 5,339,051. Although these patents produce some of the benefits associated with silicon for housing an electronic component, the package taught therein is wholly inadequate for use in aerospace and related applications. A low temperature bonding technique is employed. When subjected to the rigorous environment of outer space, the bond will melt (fail) leading to unacceptable degradation of the performance of the electronic package. Further, the proposed device involves silicon-quartz-silicon bonding, a difficult proposition at best, given the dissimilarity of the materials, made all the more risky by doubling the extent of circumferential area subject to failure by doubling the number of bonds needed. For many applications, this bond failure may not significantly impact the performance of the housed components but, as has been noted, in the demanding environment of aerospace, even the slightest degradation must be resisted as intolerable.
It is, therefore, the intent of this invention to provide a silicon package which can replace the glass bubble and provide the electronic components housed therein a hermetic environment which will perpetuate initial performance, or as close to initial performance as possible, for the duration of the life of the component. Another aspect of the invention is a method of manufacturing a surface mountable electronic component which is capable of such enduring peak performance.
The present invention comprises a hermetic package for enclosing a piezoelectric device comprising a base member made of a single crystal of metallic silicon; a cover made of a single crystal of metallic silicon; a cavity formed between the base member and the cover for receiving the piezoelectric device, the cavity providing a vacuum environment for the piezoelectric device; high temperature bonding means for securing the cover means directly to the base member. The vacuum preferably falls in the range between 1×10−5 and 1×10−11 torr, while the high temperature bonding means is preferably is performed in a temperature range of 300° C. and 800° C. While the piezoelectric device may be selected from the group of resonators, filters and surface acoustic wave devices, for aerospace and similar applications, it is preferably an ultra-stable oscillator and, most preferably, a quartz crystal. In the ultra-stable oscillator, the quartz crystal will be provided with first and second gold plated electrodes spaced from opposing surfaces thereof.
The cavity in the package is preferably formed as a recess in at least one of the base member and the cover to provide a place to house the electronic element. Most preferably, the cavity comprises a recess formed in each of the base member and the cover and may be augmented by the addition of a spacer ring. The preferred high temperature bonding means comprises brazing of the cover to the base member using gold indium eutectic alloy at 495° C. This same bonding means is preferably used to secure the piezoelectric device to at least a portion of said cavity. It is a feature of the present invention that the base member and the cover provide means of electrical contact to the piezoelectric device obviating need for lead wires within the package. This is particularly important in the aerospace applications where sealing is crucial and the absence of lead wires eliminates the possibility that the wires can adversely affect the hermetic characteristics of the device.
Another aspect of the present invention involves a hermetic package for an electronic component comprising a base member made of a material selected from the group consisting of metallic silicon, titanium, zirconia, silicon carbide, sapphire, and tantalum; a cover made of the same material as the base member; a cavity formed between the base member and the cover for receiving the electronic component, the cavity providing a vacuum environment for the electronic component; high temperature bonding means for securing the cover means directly to the base member. The vacuum has the same preferred characteristics as the first embodiment, namely, the vacuum falls in the range between 1×10−5 and 1×10−11 torr. Similarly, the high temperature bonding means is performed in a temperature range of 300° C. and 800° C., as with the previous embodiment.
Another aspect of the present invention is a method of making a surface mountable electronic component comprising the steps of providing a base member made of a material selected from a group consisting or metallic silicon, titanium, zirconia, silicon carbide, sapphire, and tantalum; providing a cover member made of the same material as the base member and defining a cavity between the base member and the cover large enough to receive the electronic component; bonding the electronic component in the cavity by a method selected from a group consisting of brazing the component in the cavity using thermo-compression bonding at a temperature approaching 500° C., laser welding, E-beam welding, anodic bonding, induction heating; bonding the cover to the base member by a method selected from a group consisting of brazing the component in the cavity using gold indium eutectic alloy at a temperature approaching 500° C., laser welding, E-beam welding, anodic bonding, induction heating, glass frit, or thermo-compression bonding.
The method also preferably includes the step of evacuating the cavity by pulling a vacuum thereon in a range of between 1×10−5 and 1×10−11 torr. For some applications, the evacuation step is performed after bonding through a small hole in one of the base member and the cover. For other applications, the evacuation step is performed before the bonding step.
Another detrimental effect for these precision electronic components is variations in temperature. It is a further aspect of the present invention to provide metallic layer bonded to an interior surface of the enclosure. The metallic layer will be selected from a group consisting of platinum and gold. Electric circuitry connects the metallic layer to control means positioned outside the enclosure so as not to interfere with the hermetic nature thereof. Either of these metals can function as a heating element within the package to maintain a more uniform ambient temperature. Platinum can function as both a heating element and a thermal sensor to determine the temperature within the enclosure.
Various other features, advantages and characteristics of the present invention will become apparent to one of ordinary skill in the art after a reading of the following specification.